Jarrod Pevitt wrote: "Hang in there Benno. Thinking of you and how strong you are dude."

Gerring's friend, Mandurah Boardriders Club president Brian Williams told the ABC he had just paddled further out into the ocean to catch a bigger wave when "all hell broke loose".

Fellow surfers pulled him from the water.

"I paddled out to them, they needed a hand and that's when I saw this fellow, he was very pale, they had him propped up on a surfboard paddling in, there was about 100m to go so we got him all the way ashore and then applied CPR," Ian Barker told reporters.

Continued below.

Related Content

Local reporter Nathan Hondros told Nine Gerring was unconscious when he was brought ashore and had had lost his leg from above the knee. "It was just terrible to watch. There was a lot of distressed people on the beach."

Paramedics worked on the mine worker on the beach for about 20 minutes before he was transferred to a waiting ambulance and taken to Peel Health Campus 13km away. He was later transferred 80km to the State Trauma Centre, accompanied by his mother.

Surf Life Saving WA tweeted earlier in the day that there was a public report of a 3.5m white shark sighted at Pyramids Beach in nearby Dawesville, about 1.8km offshore. The last shark attack in WA was in October, when there were two incidents that resulted in minor injuries.

Another shark attack occurred on Monday off California in the US.

Maria Korcsmaros suffered a semi-circular shark bite stretching from her shoulder to her pelvis, with teeth marks visible in the shredded flesh of her arm and torso.

You could see individual marks from individual teeth

Dr Phillip Rotter

Korcsmaros was training for a half-triathlon off the coast of Newport Beach when she "felt something hit her," said Dr Phillip Rotter of Orange County Global Medical Centre, where she was treated after the attack.

Surgeons repaired the massive wound with a shape that Rotter described as "obviously a mouth".

He added: "You could see individual marks from individual teeth".

The 52-year-old Korcsmaros also suffered fractured ribs, lung lesions and damage to the muscles and skin of her upper arm and abdomen. She was expected to survive, but it was too early to know whether she will regain full use of her arm.

The bite likely came from a large great white shark, said marine biologist Chris Lowe, director of Shark Lab at California State University, Long Beach.

Though difficult to draw a definitive conclusion, he said a large single bite suggests the shark is over 3m-long. A great white would be the most likely candidate lurking in Southern California waters, he added.

Korcsmaros, a mother-of-three, was not immediately sure it was a shark. Lifeguards in a boat who saw her struggling about 100m offshore found her gushing blood, doctors and fire officials said.

They saw the last part of the attack and knew she didn't get hit by a boat or watercraft, said Rob Williams, chief lifeguard of the Newport Beach Fire Department's Marine Operations Division.